The present invention relates to the use of a composition comprising a partial ester of a carboxylic acid and a polyhydric alcohol as a lubricity improver.
Over the years, commercially available fuel systems have become more and more refined. Processes have been developed to reduce sulphur levels and levels of aromatics, in particular for diesel fuel, where it has been proposed to limit the maximum distillation point of diesel fuel. However, with these pre-treatment processes lubricating properties provided by the fuel itself became worse. In relation to low density and low viscosity fuel systems such as naphta, kerosene and gas condensates, it has been observed that they show poor lubricity to metal surfaces. As a result of these poor lubrification properties, critical components such as fuel pumps and gas turbine flow dividers risk to show serious and accelerated wear upon use, which may result in premature failure. Therefore, increasing attention has been given to the addition of lubricity improving additives to fuel, with the purpose of imparting acceptable lubricating properties back to the fuel.
To balance the needs of engine makers and users, standards officials and fuels producers, the ASTM D-6079 diesel lubricity specification has been put forward, which states that a standardized wear test should not create a “wear scar” larger that 520 microns in diameter. The test makes use of a High Frequency Reciprocating Rig (HFRR) as described in D. Wei and H. Spikes, Wear, Vol. 111, N) 2, p. 217, 1986 and R. Caprotti et al, SAE paper 922183; SAE fuels and lubes, Meeting October 1992, San Fransisco USA. Although various injector and injector pump tests and several laboratory tests have been developed to measure the lubricity of diesel fuels, ASTM D6079 High Frequency Reciprocating Rig (“HFRR”) test is the most widely used and accepted.
The use of esters of long chain fatty acids as fuel lubricity additives is well known in the art, and a wide variety of additive compositions are commercially available, with the additive being optimised in function of the fuel with which it is to be used. Properties that are considered to be of importance with lubricity improvers include the following: the concentration of metal contaminants should be as small as possible, the lubricity improver should show excellent mixing ability and compatibility with the envisaged type of fuel and a minimal tendency to form gum deposits, the lubricity improver should have a low solids content and the risk to the occurrence of adverse interactions with traces of iron and other fuel additives present in the fuel, such as corrosion inhibitors, combustion improvers etc, should be negligible.
EP1216288 discloses the use of a diester of a monocarboxylic acid and a polyhydric alcohol as a lubricity improver for diesel fuel oil. The product is said to show an improved cold stability, in particular a reduced tendency to cloud or freeze at temperatures below 10° C. The monocarboxylic acid comprises a C8-C24 linear or branched chain unsaturated fatty acid or a mixture thereof, preferably a C12-C22 unsaturated fatty acid. The carboxylic acid typically has an iodine value of 40 to 180 in accordance with the AOCS Method cd-1-25. Oleic acid, linoleic acid, linolenic acid, palmitoleic acid, gadoleic acid, erucic acid and other unsaturated acids with 22 carbon atoms are particularly preferred. The unsaturated carboxylic acid component may be selected from tall oil fatty acids, soybean fatty acids, rape seed fatty acids, sunflower seed fatty acids, fish oil and cold fractionated tallow and palm oil fatty acids. The polyhydric alcohol comprises an alcohol of the general formulaCR1R2R3R4  (formula 1)or a dimer, a trimer or tetramer of the alcohol obtainable by condensation of the alcohol of formula 1. In formula 1, R1, R2, R3 are each independently of each other a C1-C12 linear or branched chain alkylol and R4 is a linear or branched chain C1-C12 alkyl or a linear or branched chain C1-C12 alkylol. The diester may be part of a mixture including higher esters such as triesters and tetraesters. It is explained that the diesters have a cold stability which is independent of the molar ratio of the fatty acid to alcohol. Lubricity efficiency is said to increase with a decreasing ratio of fatty acid to alcohol for both mono- and di-esters, whereas with monoesters the cold stability seems to decrease.
The lubricity improving compositions disclosed in EP826765 have been developed for the purpose of improving the lubricating capacity of “average petroleum distillates” for use with engines, gas turbines or diesel engines. The lubricity improvers disclosed in EP826765 include partial esters of at least one polyol and at least one monocarboxylic acid. Suitable polyols include propanediol-1,2, propanediol-1,3, the butanediol-1,3, butanediol-1,4, neopentyl-glycol, glycerol, diglycerol, polyglycerols, trimethylolpropane and pentaerythritol and mixtures containing two or more thereof. Suitable monocarboxylic acids include those having between 4 and 24 carbon atoms, for example butyric, caproic, caprylic, capric, lauric, myristic, palmitic, palmitoleic, stearic, oleic, linoleic, ricinoleic, arachidic, gadoleic, behenic, erucic and lignoceric acid. Particularly preferred are di-esters of glycerol or di-esters of glycerol in a mixture with mono esters. The examples disclose mixtures of 37% respectively 55% of monoglycerides and 62% respectively 45% of diglycerides.
DE19614722 discloses lubricity improvers with an improved cold stability, which mainly consist of mixtures of mono- and diesters of unsaturated fatty acids with 16-22 carbon atoms and at least two different polyols with 3-10 carbon atoms and 3-6 hydroxyl groups. Preferred polyols are trimethylolpropane, pentaerythritol and dipentaerythritol in a molar of 1:3 to 3:1. Suitable fatty acids include oleic acid, linoleic acid, linolenic acid.
To date, attempts to improve lubrication properties of esters of fatty acids and polyols, mainly concentrated on optimising the fatty acid composition. Applications of diglycerol esters remained limited until today, and were mostly situated in the field of cosmetics.
The present invention seeks to further improve lubricating properties provided by fatty acid ester compositions.